Re: [PATCH v6 17/25] iommu/arm-smmu-v3: Implement iommu_sva_bind/unbind()

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Fri, May 01, 2020 at 05:15:52AM -0700, Christoph Hellwig wrote:
> > @@ -432,6 +432,7 @@ config ARM_SMMU_V3
> >  	tristate "ARM Ltd. System MMU Version 3 (SMMUv3) Support"
> >  	depends on ARM64
> >  	select IOMMU_API
> > +	select IOMMU_SVA
> >  	select IOMMU_IO_PGTABLE_LPAE
> >  	select GENERIC_MSI_IRQ_DOMAIN
> 
> Doesn't this need to select MMU_NOTIFIER now?

Yes, will fix

> > +	struct mmu_notifier_ops		mn_ops;
> 
> Note: not a pointer.
> 
> > +	/* If bind() was already called for this (dev, mm) pair, reuse it. */
> > +	list_for_each_entry(bond, &master->bonds, list) {
> > +		if (bond->mm == mm) {
> > +			refcount_inc(&bond->refs);
> > +			return &bond->sva;
> > +		}
> > +	}
> > +
> > +	mn = mmu_notifier_get(&smmu_domain->mn_ops, mm);
> > +	if (IS_ERR(mn))
> > +		return ERR_CAST(mn);
> 
> Which seems to be to avoid mmu_notifier_get reusing notifiers registered
> by other arm_smmu_master instance right?

Yes, although I'm registering a single mmu notifier per (domain, mm) pair,
not (master, mm), because the SMMU driver keeps one set of PASID tables
per IOMMU domain.

> Either you could just use plain old mmu_notifier_register to avoid
> the reuse.  Or we could enhance the mmu_notifier_get to pass a private
> oaque instance ID pointer, which is checked in addition to the ops,
> and you could probably kill off the bonds list and lookup.

Going back to mmu_notifier_register() seems better for now. I don't want
to change the core APIs just for this driver, because it's likely to
change again when more hardware starts appearing and we optimize it.

Thanks,
Jean